A known technique for obtaining vapour phase spectra involves heating up a solid sample of a compound, estimating the concentration of the resulting vapour using the Clausius-Clapeyron relation, and measuring the spectrum of the vapour. This results in a measured spectrum for a given vapour concentration.
A known way to implement this approach is to coat an inert base such as small silica beads with a layer of the sample compound. The thin layer can then undergo a controlled vaporisation and its spectrum be measured. This method is suitable for unstable compounds which must be used in small quantities to prevent them from decomposing, potentially explosively, and therefore reduces the risks associated with heating up a solid sample of an unstable compound. However, the emission rate is very low with this technique, and for volatile compounds it is difficult to create a stable, thin condensed phase coating because the compound evaporates.
To reduce evaporation of volatile compounds and to desensitise unstable compounds, a sample can be mixed with a solvent to form a dilute solution. A combined vapour phase spectrum of the mixture is measured and the sample spectrum is extracted from the combined spectrum by post-analysis. However, a simple subtraction of the pure solvent spectrum is not sufficient because of complex molecular interactions between the dissolved sample compound and the solution, and post-analysis techniques do not adequately account for this. Furthermore, to measure the separate vapour concentrations of the sample compound and the vaporised solvent, gas chromatography mass spectrometry is used but this introduces inaccuracies.
To desensitise an unstable sample compound without using a solvent, the sample may be deactivated. However, this chemically changes the sample, which is likely to create differences in the spectrum that are difficult to predict or recognise.
Furthermore, the quantitative accuracy of any technique using the Clausius-Clapeyron relation is restricted because of condensation and adsorption effects which move the true vapour concentration away from the calculated value, and also because assumptions inherent in the Clausius-Clapeyron relation limit its accuracy.
Another known technique for obtaining pure vapour phase spectra is to extrapolate from condensed matter phase spectra. The extrapolation is not straightforward, however, because intermolecular interactions present only—or to a greater extent—in the condensed phase are difficult to predict with quantitative accuracy. Also, if the vapour phase is less stable and readily dissociates into decomposition compounds with their own absorption bands, this can be difficult to predict from condensed phase data alone.
The present invention aims to address one or more of the deficiencies associated with the prior art.